glTF model support

[appgurueu]

• Goal of the PR: Add glTF model support.
• How does the PR work? The Minetest side of things is trivial; just add glTF to the list of supported formats, and add some glTF test models & entities. (Also, I renamed an Irrlicht method which was wrongly named. That is a separate commit.)
• Does it resolve any reported issue? Closes Support for more modern mesh format(s) (.gltf) #9673.
• Does this relate to a goal in the roadmap? glTF support is on the roadmap.
• If not a bug fix, why is this PR needed? What usecases does it solve? Has been explained in the issue already. Still, top points (IMO) are:
  • Lowers the bar to model creation.
  • Provides a solid basis for adding features beyond what .b3d / .x support (better materials, animations, etc.)
  • Brittle .b3d / .x toolchains need not be relied on anymore.

Depends on:

1. Static glTF mesh loading irrlicht#200
2. Fix gltf static mesh loading issues JosiahWI/irrlicht#14
3. Complete glTF loader, add animation support JosiahWI/irrlicht#12

(WIP, PRs 2 and 3 are pending review by Josiah.)

To do

• Move tiniergltf to a separate repo. (I'm willing to maintain this.)
• Sparse accessor support (and related accessor refactoring).
• Fix Irrlicht PR MinGW builds (either by bundling the jsoncpp amalgamation like Minetest or getting sfan to add Jsoncpp to the available system packages fixed by forking jsoncpp and fetching it via FetchContent).
• Thorough testing and code review, resulting in fixes and refactorings.
• Warnings, like not using all nodes in a scene or embedding images (which we won't read).
• Profile to be sure that the loader has acceptable performance (highly likely IMO).
• (low priority, maybe out of scope, but also low effort) add .glb support for smaller file sizes.
• (very low-priority, likely out of scope) rewrite the base64 implementation for fun and profit.

How to test

1. Check out & build https://github.com/appgurueu/irrlicht/tree/feat/gltf-loader-tinier-fixmes-animated.
2. Run the Irrlicht unittests: ctest --output-on-failure (inside the irrlicht dir).
3. Check out & build this PR.
4. Spawn the test entities.
5. 👁️ 👄 👁️

Screencast.from.09-01-24.00.47.45.webm

Trivia

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The work to make this possible was done by what I would call Minetest's "inofficial gltf working group" - Josiah, Jordan, and me. The spider model used for testing was made by archfan.

[sfan5]

Rebase needed after IrrlichtMt merge.

[rubenwardy]

Too WIP, removed from 5.9 milestone. Please work on this as gltf is a very desired feature